The best time is when you are healthy. Then he can learn about you. The two of you can work together to create a plan for you to improve your health and maintain a disease-free life for 100 years or more. This is the best time.
In fact, in my opinion a doctor should be paid to keep you healthy. Isn't that what a "Health Maintenance Organization" is... anyway? Keeping you healthy is what he is good for. The best kind of doctor is one who keeps you out of the hospital, away from drugs, and completely surgery free.
Also, when you are healthy you have the time to interview many doctors until you find the one you like, who you can talk to, who really cares about you. Find an open-minded doctor who doesn't smoke and is not overweight. That's my advise. Find a doctor you can partner with as an equal; even exercise with and share discoveries about health and nutritional supplements with.